Depends on how you see it, you can consider multiple different options:
In my opinion: 3, Aizen, Muramasa (zampaktous filler), Jin (bounts filler), Ganryū (movie Memories of Nobody), Kusaka (movie, The Diamond Dust Rebellion), Shizuku and Homura (movie, Fade to Black), that guy from hell (movie Hell Chapter) and that guy from the princess arc filler wich i don't remember the name.
Other chars u can also see as main villains:
-the hollow version of Ichigo (often called as Ogihci, but as said by him he has no name), but still it's just an inner conflict in Ichigo
-Gin and Tosen, as they are the main alies of Aizen
-Any of the espada
-Yamamoto, in the initial episodes the soul society was Ichigo's enemy and as he's the Leader
-*Warning spoilers on this answer, if u just watch the anime may not read* Urahara, as said by Kugo Ginjo that he's no good and been using Ichigo
Gin isn't an ally of Aizen, quite the opposite either
Tosen was just an easily manipulated blind dude
Well you first Par of answers was what i was thinking so
What strange thing happened to yammi that shocked every1 after he released?
Correct sry for the absence.
What is the name of Ichigo's Greatest( as of now lol) technique